Heat-resistant alloys



No Drawing. Application 9, 19 54 SerialNo.,415,164 1 4 priority,application France March is, 1953 4 Claims. (01. 15-204 This inventionrelates to a method of preparing hard, tough alloysable to preservetheir hardness and toughness up toitemperatures as highas l200 C. I. Thedevelopment of gas turbine engines could not be achieved until alloyswere available which were sufiiciently refractory to heat, that is tosay,alloys which, at elevated temperatures, remain tough, undeformable,and resistant to deterioration by oxidation.- Even'at present anyincrease in the potentialities of turbine engines such as power,reliable running, duration of service, and low maintenance cost, isclosely linked to theimprovements which research enables-to be made in-,thepossibility-of raising the temperature. at which materials.subjected to certain stresses can be used. v .f I The study of variouscompositions of alloys ofcobalt, nickel, chromium, iron, molybdenum,carbon, etc. has formed the subject of a great deal of investigation.The lawsrelating heat properties to compositions and to methods ofproduction have been determined, and it can be assumed that the bestrefractory alloys which can be made from the elements mentioned aboveare now known. Now the efficiency of turbine engines could besubstantially increased still further if the turbines were capable ofworking at a temperature considerably higher-than those which can betolerated by the best of these latter alloys and which is of the orderof 800 C. f

It has been suggested that recoursegshould be had to metallic-carbides,"which are justly reputed for. retaining their toughness atanelevated temperature, and of these, titanium carbide has receivedparticularv attention because of its combination of properties.v Itislindeedremarkable for its low specific weight, substantiallyequal 'to.5 g./cm. which, in comparison with other refractory alloys, opens thepossibility of a considerable reduction in the inertia stresses to whichrapidly moving partsare sub jected, for its high heat modulus, andfor'itsiresistance to oxidation; moreoverits cost is moderate, far belowthat of other refractory carbides which also have the disadvantage ofbeing heavy and-oxidizableJ Unfortunately, up to date .no one hassucceeded in making compact bodies of titanium'carbide, which werenon-brittle and free from porosity. I Fusion takes place at too high atemperature to beconsidered. Nor is. the process of compression andcalcination suitable, because thegrains of titanium carbide, deprivedofall plasticity, only: remain in' contact at a few points, however greatthe moulding pressure may be, and the alloy. finallyobtained is brittleand porous.

Attempts have been made to agglomeratekthe grains of carbide byconventional methods using;a metal. of the iron group, and to overheatthe product in a vacuum so as to evaporate the bonding metal, but apartfrom thefact that the operation is diflicult and expensive, the actualseparation of the bonding metals detracts from the cohesion of thealloy. If the elimination of the-bonding metal is incomplete, theplasticity; when but of. the

intergranular cement which is left confers such a capac ity fordeformation on the material that there is practically no point insubstituting these alloys for thoseconsisting of nickel, chromium,cobalt, iron, etc, without titanium carbide. 2 I l According to thepresent invention, a method of prepa? ration has now been discovered, bymeans of which alloys are obtained having a, remarkable combination ofrnef chanical and physico-chemicalproperties whichhave not yet beenunited in a single product, which alloys-are suit: able for any partwhich is exposed locally orentirely to an elevated temperature and whichis, reguired, under these conditions, to tolerate considerable andprolonged stresses without deformation or deterioration; a The method ofpreparation, which forms the subject of the invention, relates to alloysof carbon andtitanium in which the carbon content comprises between 13and 18%. In carrying out the process, the raw materials are titaniumhydrideand pure titanium carbide in which the carbon and the titaniumare in equiatomic proper-Q tions, the proportion of titanium hydride inthe mixture amounting to between 10 and 35%. The two'substances areintimately mixed in a suitable proportion in, aball mill; the mixturethus prepared is compressed in a mould of cylindrical or prismatic shapeunder suflicient pressure 'to'obtain, after extraction from the mould, ablock which is sufliciently coherent to be handled without crumbling.The compressed tablets thus obtainedare heated slowly in an atmosphereof hydrogen-up to about l,000 'C." In'the course of this heating,-ithetitaniurn hydride is dissociated and liberates its hydrogen and; asaresult, the compressed tablets break up, at'least in those alloysaccording to the invention which are prepared from the highestproportions of titanium hydride, The tablets are thenremovecl fromthefurnace, the ,pow: der which constitutes them' is dispersed by briefcrushing and again subjected to compression, though this time in thefinal shape required, bearing in mind the variations in dimension whichwill take place during the heating which follows. The shaped partsacquire their cohesion by heating in an atmosphere of hydrogen to atemperature above 1.800 (3., atemperature of betweenZDOO and 2.200 C.,maintained for a period lasting between 10 minutes and onehour, beingsuitable. L When hydrogen is used as an atmosphere for calciningfurnaces, it is ditficult to avoid a variation inthe composition ofthe'surface regions of the parts, whether decarburization takes place,or cementation (addition of carbon) through the methane which resultsfrom the action of the hydrogen on the resistors or crucibles when madeof graphite. This disadvantage may be overcome by coating the partssubjected to calcining with a thin layer of aluminium or better still bycarrying out the calcining in an atmosphereof rare gas, for exampleargon. For reasons of economy, the air originally contained in thefurnace may be blown out bya current of hydrogen which is retained untilthe furnace has reached a temperatureof 1.000 C. Argon is thensubstituted for the hydrogemwhich is gradually driven out as thetemperature rises, the highest temperature being maintained in anatmosphere of substantially pure argon and the small quantities ofhydrogen remaining not having any noticeable effect. I, V The excellentmoulding capacity of the powder is unexpected and appears to result fromthe coating of the grains of carbide with the titanium, resulting fromthe dissociation of the hydride and from. the good distribution of thetitanium in the surface layers of ,the carbide grains. ,Thus anexcellent compactness, is obtained by cold moulding of the powder, andan excellent subsqu'eiit' alciiiihg. i

In alloys having a high carbon content, within the limits of theinvention, for example, those which are made from titanium hydride and90% titanium carbidej the separation of hydrogen which takes placeupto"1.000 C. is sufficiently moderate, if heating is carried out"sloilvly, for the compressed tablet subjected thereto to" retain itsshape, without faults or cracks occurring. It is therefore unnecessaryto remove the compressed tablet from the furnace and the heating canbecontinued up to the temperature necessary for the final cohesion.Needless tosay, when a singleheating is sufii cient, the moulding is domin the final shape, making allowance forshlinkage. A speeifiebut non-limiting example of preparation according to the' invention is givenbelow.

"-800 giof pure titanium carbide CTi and 200 g. of titanium hydride areintroduced into a'ball mill filled withtungsten carbide-balls, togetherwith just sufficient volumeof petroleum spirits for the pulverulent massto assume the consistency of cream, thepetroleum spirits maintaining thedispersion of the powder during the pulverizin'g and in particularencouraging the intimateness'ofthe mixture. The pulverization lasts for10 hours, afterwhich theliquid suspension of the powders is removed fromthe ball mill; the petroleum spirits are separated by evaporationcarried out at about 100, preferably in an atmosphere of hydrogen. Fromtheclry powder (l-compressed tablet is made under a' pressure of 10kgJmrn. and in the following operation'is gradually heated from roomtemperature to l.000 a furnace traversed by a current of hydrogen. Themass, whichhas again become pulverulent, is removed from the furnace andthe agglomerates which remain are dispersed :by blows orby briefpulverization. The powder obtained is placedin a mould of the shape ofthe desired part, the dimensions of the mould being 13.5% greater thanthose which are to be obtained finally. For? part ofregular prismaticshape, very well shaped homogeneous compressed tablets, having clearlimits, are obtained under apressure of 12 kg./mm. These compressedtablets are heated under a current of hydrogen in a graphite resistancefurnace in such a manner that they are maintainedfor 20 minutes at 2.050C.

The parts finally obtained have a density of 4.85 g ns/onu andtheirhardness is 1.500 kg./mrn. Vickers. "iheirresistance to oxidation issuch that they can be in contact with the for several days at atemperature of 900 without their; hardness or theirtoughnsesdeteriorating, and without undergoing any change other than a slightvariation in the tint of the surface which becomes a little less clear.Microscopic examination of the alloy obtained with a magnification of1.500 diameters, after polishing with diamond powder, and etching with asolution of potassium ferricyanide, shows that it consists ofhomogeneous fine grains with very fine and sinuous contours. Theagglomerating titanium has therefore diffused in the carbide grains inthe course of the high temperature calcining, thus leading to apolycrystalline substance'without any intergranular binding agent. Theimpossibility of intergranular slip resulting from this fact .andfromthesinuous shape of the contours explains whynoplasticity appearsin thealloy up to veryhigh temperatures, for example 1.400 C. At least 1.800has t-otbe. reached for a certain plasticity to appear, resulting frp nthe relaxation of the interatomic bonds inside the grains.

When. parts are manufactured whichcould not be withdrawn frjom a mould,or the dimensions of which are too great forlahomogeneous compactness tobe obtained by moulding, i't iefpossibleQafter the dissociation of thehydride, to rnouldjthe powder in the shape of a prismatic bar to subjectthis to calcining at a temperature just sufficient to permit machininginto the final shape, anclwhen high temperature;

this has been done, to proceed to the final calcining at a The alloysthus prepared, which have more than about 14% of carbon are perfectlystable structurally at all temperatures.

As a result of their very low density, their hardness and their highmodulus of elasticity at a high temperature, and of their resistance todet ration by oxidation, the alloys prepared form suitable in rials'forparts such as turbine blades and exhaust "valves for internalcombustionengines pperating aha high temperature and the rapid displacementgffwhichsi to heavy stresses. These products can be used for allparts which are required 'to'be tough at a high temperature.

These alloys are much less expensive than the usual alloys having-atungsten carbide base, and can be substituted for them in certaincutting or friction tools.

l. A method of preparing hard, tough alloys having a density of lessthan 5.0 gm./cm. and having the property of retainingtheirhardness andtoughness up to atemperature of '1200' C which comprises intimatelycombining titanium hydride and titanium carbide into 5 a pulverulentmixture, the carbon and the titanium being in substantially equiatoinicproportions, and the percentage of titanium hydride in the mixture lyingbetween 10% and 35 compressing the'pulverulent mixture into briquettesof'sufiicient cohesion to be handled, heating the briquettes toatemperature fsufficient to eliminate hydrogencompletely from thehydride, disintegrating the thus-heated briquettes," compressing-theparticles of the disintegrated briquettes in a mold, and heating thematerial in a mold in an inertatmosphre to a temperaturebetween 1900 and2300 C., whereby to form a polycrystalline substance consistingofhomogeneous fine grains free from "any intergranular binding agent.

2. A method of 'preparing'hard, tough alloys having a density of lessthan 5.0 gin/cm. andha'ving the property of retaining their hardness andtoughness up to a temperature of l2 00 C; which comprises intimatelycombining titanium hydride and titanium carbide into a pulverulentmixture, the carbon and 'the titanium being in substantially equiatornicproportions, and the percentage of titanium hydride in themixturelyingbetween 10% and 35%, compressing the pulverulent mixture into a mold,heatingthe material in the mold slowly from room temperature to aboutl000, C. to eliminatehydrogen completely from the hydride, and thenheating the material in amold in an inert atmosphere 'to a "temperaturebetween 1900 and 2300" CL, \ivhereby to form a polycrystalline substanceconsisting ofjhomogeneous fine grains free from any inter granularbinding agent. i

3.;A method of preparing hard, tough alloys havinga densityof less than5.0 gum/cm. and having the property of retainingtheir hardness andtoughness up to a ternperature of 12 00 C. which comprises intimatelycombining titanium hydride and titanium carbide into a pulverulentmixture, the carbon and the titanium being in substantially equiatomicproportions, and the percentage of titanium'hydride' in the mixturelying between 10% and 35%, compressing the pulverulent mixture intobriquettes f sufiicient cohesion to behandled, heatingthe briquettestoatemperature suflieientto eliminate hydrogen coinpletelyfroinfthehydride, disintegrating the thus heated briquettescompressing theparticles of the, disintegrated briquettes in amold, heating thematerial in amoldin an inert atmosphere to a temperature of about 1300and then heatingsaid material to a temperature between 1900 and 2300C.," whereby to form apolycrystalline substance consisting ofhomogeneous fine grains free from any. intergranular} binding agent.

4. A method of preparin hard, tough alloys having a density of lessthan"5.0 gmZ/cmi and having the property of retaining their hardness andtoughness up to temperature of 1200 C., which comprises intimatelycombining titanium hydride and titanium carbide into a pulverulentmixture, the carbon and the titanium being in substantially equiatomicproportions, and the percentage of titanium hydride in the mixture lyingbetween 10% and 35%, compressing the pulverulent mixture into a mold,heating the material in the mold slowly from room temperature to about1000" C. to eliminate hydrogen completely from the hydride, heating saidmaterial in an inert atmosphere to about 1300 C., and then heating thematerial in a mold in an inert atmosphere to a temperature between 1900and 2300 C., whereby to form a polycrystalline substance consisting ofhomogeneous fine grains free from any intergranular binding agent.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Nelson: Journal of Electrochemical Society, vol. 98, pp.465-473, December 1951.

1. A METHOD OF PREPARING HARD, TOUGH ALLOYS HAVING A DENSITY OF LESSTHAN 5.0 GM./CM.3 AND HAVING THE PROPERTTY OF RETAINING THEIR HARDNESSAND TOUGHNESS UP TO A TEMPERATURE OF 1200* C., WHICH COMPRISESINTIMATELY COMBINING TITANIUM HYDRIDE AND TITANIUM CARBIDE INTO APULVERULENT MIXTURE, THE CARBON AND THE TITANIUM BEING IN SUBSTANTIALLYEQUIATOMIC PROPORTTIONS, AND THE PERCENTAGE OF TITANIUM HYDRIDE IN THEMIXTURE LYING BETWEEN 10% AND 35%, COMPRESSING THE PULVERULENT MIXTUREINTO BRIQUETTES OF SUFFICIENT COHESION TO BE HANDLED, HEATING THEBRIQUETTES TO A TEMPERATURE SUFFICIENT TO ELIMINATE HYDROGEN COMPLETELYFROM THE HYDRIDE, DISINTEGRATING THE THUS-HEATED BRIQUETTES, COMPRESSINGTHE PARTICLES OF THE DISINTEGRATED BRIQUETTES IN A MOLD, AND HEATING THEMATERIAL IN A MOLD IN AN INERT ATMOSPHERE TO A TEMPERATURE BETWEEN 1900AND 2300* C., WHEREBY TO FORM A POLYCRYSTALLINE SUBSTANCE CONSISTING OFHOMOGENEOUS FINE GRAINS FREE FROM ANY INTERGRANULAR BINDING AGENT.